MercuryConverter/Overrides/Expander.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">

  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Orientation="Vertical" Spacing="20" Width="350" Height="600">
        <Expander HorizontalAlignment="Stretch" ExpandDirection="Up" Header="Expand Up">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander HorizontalAlignment="Stretch" ExpandDirection="Down" Header="Expand Down">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander HorizontalAlignment="Stretch" ExpandDirection="Left" Header="Expand Left">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander HorizontalAlignment="Stretch" ExpandDirection="Right" Header="Expand Right">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander HorizontalAlignment="Stretch" ExpandDirection="Up">
          <Expander.Header>
            <Grid ColumnDefinitions="*, Auto">
              <TextBlock Grid.Column="0" Text="Control" />
              <TextBlock Grid.Column="1" Text="Header" />
            </Grid>
          </Expander.Header>
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Up" Header="Rounded" CornerRadius="25">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <!-- Shared header/content -->
  <x:Double x:Key="ExpanderMinHeight">48</x:Double>

  <!-- Header -->
  <HorizontalAlignment x:Key="ExpanderHeaderHorizontalContentAlignment">Stretch</HorizontalAlignment>
  <VerticalAlignment x:Key="ExpanderHeaderVerticalContentAlignment">Center</VerticalAlignment>
  <Thickness x:Key="ExpanderHeaderPadding">16,0,0,0</Thickness>
  <Thickness x:Key="ExpanderHeaderBorderThickness">0</Thickness>
  <Thickness x:Key="ExpanderChevronBorderThickness">0</Thickness>
  <Thickness x:Key="ExpanderChevronMargin">20,0,8,0</Thickness>
  <x:Double x:Key="ExpanderChevronButtonSize">32</x:Double>

  <!-- Content -->
  <Thickness x:Key="ExpanderContentPadding">8</Thickness>
  <Thickness x:Key="ExpanderContentLeftBorderThickness">0,0,0,0</Thickness>
  <Thickness x:Key="ExpanderContentUpBorderThickness">0,0,0,0</Thickness>
  <Thickness x:Key="ExpanderContentRightBorderThickness">0,0,0,0</Thickness>
  <Thickness x:Key="ExpanderContentDownBorderThickness">0,0,0,0</Thickness>

  <ControlTheme x:Key="FluentExpanderToggleButtonTheme" TargetType="ToggleButton">
    <Setter Property="Background" Value="{DynamicResource ExpanderHeaderBackground}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ExpanderHeaderBorderBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ExpanderHeaderBorderThickness}" />
    <Setter Property="Foreground" Value="{DynamicResource ExpanderHeaderForeground}" />
    <Setter Property="Padding" Value="{DynamicResource ExpanderHeaderPadding}" />
    <Setter Property="HorizontalAlignment" Value="Stretch" />
    <Setter Property="HorizontalContentAlignment" Value="{DynamicResource ExpanderHeaderHorizontalContentAlignment}" />
    <Setter Property="VerticalContentAlignment" Value="{DynamicResource ExpanderHeaderVerticalContentAlignment}" />
    <Setter Property="Template">
      <ControlTemplate>
        <Border x:Name="ToggleButtonBackground"
                CornerRadius="{TemplateBinding CornerRadius}"
                Background="#00ffffff"
                BorderBrush="{TemplateBinding BorderBrush}"
                BorderThickness="{TemplateBinding BorderThickness}">
          <Grid x:Name="ToggleButtonGrid"
                ColumnDefinitions="*,Auto">
            <ContentPresenter x:Name="PART_ContentPresenter"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                              Foreground="{TemplateBinding Foreground}"
                              Margin="{TemplateBinding Padding}"/>
            <Border x:Name="ExpandCollapseChevronBorder"
                    Grid.Column="1"
                    Width="{DynamicResource ExpanderChevronButtonSize}"
                    Height="{DynamicResource ExpanderChevronButtonSize}"
                    Margin="{DynamicResource ExpanderChevronMargin}"
                    CornerRadius="{DynamicResource ControlCornerRadius}"
                    BorderBrush="{DynamicResource ExpanderChevronBorderBrush}"
                    BorderThickness="{DynamicResource ExpanderChevronBorderThickness}"
                    Background="">
              <Path x:Name="ExpandCollapseChevron"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center"
                    RenderTransformOrigin="50%,50%"
                    Stretch="None"
                    Stroke="{DynamicResource ExpanderChevronForeground}"
                    StrokeThickness="1">
                <Path.RenderTransform>
                  <RotateTransform />
                </Path.RenderTransform>
              </Path>
            </Border>
          </Grid>
        </Border>
      </ControlTemplate>
    </Setter>

    <Style Selector="^[Tag=expanded] /template/ Path#ExpandCollapseChevron">
      <Style.Animations>
        <Animation FillMode="Both" Duration="0:0:0.0625">
          <KeyFrame Cue="100%">
            <Setter Property="RotateTransform.Angle" Value="180" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>

    <Style Selector="^[Tag=collapsed] /template/ Path#ExpandCollapseChevron">
      <Style.Animations>
        <Animation FillMode="Both" Duration="0:0:0.0625">
          <KeyFrame Cue="0%">
            <Setter Property="RotateTransform.Angle" Value="180" />
          </KeyFrame>
          <KeyFrame Cue="100%">
            <Setter Property="RotateTransform.Angle" Value="0" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>

    <!-- PointerOver -->
    <Style Selector="^:pointerover /template/ Border#ToggleButtonBackground">
      <Setter Property="Background" Value="{DynamicResource ExpanderHeaderBackgroundPointerOver}" />
      <Setter Property="BorderBrush" Value="{DynamicResource ExpanderHeaderBorderBrushPointerOver}" />
    </Style>
    <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="Foreground" Value="{DynamicResource ExpanderHeaderForegroundPointerOver}" />
    </Style>
    <Style Selector="^:pointerover /template/ Border#ExpandCollapseChevronBorder">
      <Setter Property="Background" Value="" />
      <Setter Property="BorderBrush" Value="{DynamicResource ExpanderChevronBorderBrushPointerOver}" />
    </Style>
    <Style Selector="^:pointerover /template/ Path#ExpandCollapseChevron">
      <Setter Property="Stroke" Value="{DynamicResource ExpanderChevronForegroundPointerOver}" />
    </Style>

    <!-- Pressed -->
    <Style Selector="^:pressed /template/ Border#ToggleButtonBackground">
      <Setter Property="Background" Value="{DynamicResource ExpanderHeaderBackgroundPressed}" />
      <Setter Property="BorderBrush" Value="{DynamicResource ExpanderHeaderBorderBrushPressed}" />
    </Style>
    <Style Selector="^:pressed /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="Foreground" Value="{DynamicResource ExpanderHeaderForegroundPressed}" />
    </Style>
    <Style Selector="^:pressed /template/ Border#ExpandCollapseChevronBorder">
      <Setter Property="Background" Value="" />
      <Setter Property="BorderBrush" Value="{DynamicResource ExpanderChevronBorderBrushPressed}" />
    </Style>
    <Style Selector="^:pressed /template/ Path#ExpandCollapseChevron">
      <Setter Property="Stroke" Value="{DynamicResource ExpanderChevronForegroundPressed}" />
    </Style>

    <!-- Disabled -->
    <Style Selector="^:disabled /template/ Border#ToggleButtonBackground">
      <Setter Property="Background" Value="{DynamicResource ExpanderHeaderBackgroundDisabled}" />
      <Setter Property="BorderBrush" Value="{DynamicResource ExpanderHeaderBorderBrushDisabled}" />
    </Style>
    <Style Selector="^:disabled /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="Foreground" Value="{DynamicResource ExpanderHeaderForegroundDisabled}" />
    </Style>
    <Style Selector="^:disabled /template/ Border#ExpandCollapseChevronBorder">
      <Setter Property="Background" Value="" />
      <Setter Property="BorderBrush" Value="" />
    </Style>
    <Style Selector="^:disabled /template/ Path#ExpandCollapseChevron">
      <Setter Property="Stroke" Value="{DynamicResource ExpanderChevronForegroundDisabled}" />
    </Style>

  </ControlTheme>

  <ControlTheme x:Key="FluentExpanderToggleButtonUpTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 7 L 7 0 L 14 7" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="FluentExpanderToggleButtonDownTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 0 L 7 7 L 14 0" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="FluentExpanderToggleButtonLeftTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 7 0 L 0 7 L 7 14" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="FluentExpanderToggleButtonRightTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 0 L 7 7 L 0 14" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="{x:Type Expander}" TargetType="Expander">
    <Setter Property="IsTabStop" Value="False"/>
    <Setter Property="MinWidth" Value="{DynamicResource FlyoutThemeMinWidth}" />
    <Setter Property="MinHeight" Value="{DynamicResource ExpanderMinHeight}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ExpanderContentBorderBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentDownBorderThickness}" />
    <Setter Property="Padding" Value="{DynamicResource ExpanderContentPadding}" />
    <Setter Property="HorizontalAlignment" Value="Left" />
    <Setter Property="VerticalAlignment" Value="Center" />
    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
    <Setter Property="Template">
      <ControlTemplate>
        <DockPanel MinWidth="{TemplateBinding MinWidth}"
                   MaxWidth="{TemplateBinding MaxWidth}">
          <ToggleButton x:Name="ExpanderHeader"
                        MinHeight="{TemplateBinding MinHeight}"
                        CornerRadius="{TemplateBinding CornerRadius}"
                        IsEnabled="{TemplateBinding IsEnabled}"
                        Content="{TemplateBinding Header}"
                        ContentTemplate="{TemplateBinding HeaderTemplate}"
                        IsChecked="{TemplateBinding IsExpanded, Mode=TwoWay}" />
          <Border x:Name="ExpanderContent"
                  IsVisible="{TemplateBinding IsExpanded, Mode=TwoWay}"
                  Background="{TemplateBinding Background}"
                  BorderBrush="{TemplateBinding BorderBrush}"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  MinHeight="{TemplateBinding MinHeight}"
                  HorizontalAlignment="Stretch"
                  VerticalAlignment="Stretch"
                  Padding="{TemplateBinding Padding}">
            <ContentPresenter x:Name="PART_ContentPresenter"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Foreground="{TemplateBinding Foreground}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
          </Border>
        </DockPanel>
      </ControlTemplate>
    </Setter>

    <Style Selector="^ /template/ ToggleButton#ExpanderHeader:pressed">
      <Setter Property="RenderTransform" Value="{x:Null}" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader, ^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="VerticalAlignment" Value="Stretch" />
    </Style>

    <Style Selector="^:expanded /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Tag" Value="expanded" />
    </Style>

    <Style Selector="^:not(:expanded) /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{Binding $parent[Expander].CornerRadius}" />
      <Setter Property="Tag" Value="collapsed" />
    </Style>
    <Style Selector="^:expanded:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource BottomCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:up /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource TopCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource TopCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:down /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource BottomCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource RightCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:left /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource LeftCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource LeftCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:right /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource RightCornerRadiusFilterConverter}}" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Right" />
    </Style>
    <Style Selector="^:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Bottom" />
    </Style>
    <Style Selector="^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Left" />
    </Style>
    <Style Selector="^:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Top" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{DynamicResource FluentExpanderToggleButtonLeftTheme}" />
    </Style>
    <Style Selector="^:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{DynamicResource FluentExpanderToggleButtonUpTheme}" />
    </Style>
    <Style Selector="^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{DynamicResource FluentExpanderToggleButtonRightTheme}" />
    </Style>
    <Style Selector="^:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{DynamicResource FluentExpanderToggleButtonDownTheme}" />
    </Style>

    <Style Selector="^:left /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentLeftBorderThickness}" />
    </Style>
    <Style Selector="^:up /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentUpBorderThickness}" />
    </Style>
    <Style Selector="^:right /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentRightBorderThickness}" />
    </Style>
    <Style Selector="^:down /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentDownBorderThickness}" />
    </Style>
  </ControlTheme>
</ResourceDictionary>